A currently accepted theory of ageing blames the irreversible changes in cell machinery and reduced efficiency of metabolic processes on the detrimental effects of free radicals and other reactive oxygen species (ROS) or reactive nitrogen species (RNS) which are normally present in the cell as part of the respiratory process. ROS and RNS oxidize/nitrate DNA, proteins, lipids and other cell components. Of these, protein oxidation, which converts arginine, lysine, threonine, thryptophan and proline into corresponding carbonyl compounds, cannot be repaired by proteases after a certain threshold number of amino acid residues have been oxidized.
The damaged protein loses its catalytic or structural activity, but proteases are unable to disintegrate heavily carbonylised strands, so that the damaged species accumulate and aggregate, clogging up cellular passages. This rust-like process gradually wears down all cellular mechanisms, slowing everything down and ultimately causing cellular death.
Apart from ageing, many diseases such as Alzheimer's, Parkinson's, dementia, cataract, arthritis, chronic renal failure, acute repiratory syndrome, cystic fibrosis, diabetes, psoriasis and sepsis, to give a few examples, are associated with increased protein carbonylation. Typically, physiological levels of protein carbonyls are at around 1 nmol/mg protein, whereas pathological levels go to 8 nmol/mg and above.
For the two molecules involved in the process of oxidative damage of proteins, i.e. an oxidizer and its substrate, the oxidizer has been the subject of many studies aiming at neutralizing or removing it by means of increasing the number of antioxidants (vitamins, glutathione, peptides or enzymes). The substrate, e.g. amino acid (AA) residues which are converted into carbonyls, has received less attention.
One common feature of all the AA residues (except proline) vulnerable to carbonylation is that they belong to the group of essential AAs, which cannot be synthesized by vertebrata and should be ingested, e.g. consumed with food. The group includes phenylalanine, valine, tryptophan, threonine, isoleucine, methionine, histidine, arginine, lysine and leucine (arginine is essential for children of up to 5 years of age).
Oxidation of both Arg and Lys by ROS yields aminoadipic semialdehyde and proceeds through sequential replacement of ω-hydrogens with hydroxyls. Oxidation of Lys, Arg, Trp, Thr, Phe and His is shown in FIG. 1. Side-chains undergo the same transformations if these AAs are part of polypeptides/proteins. Other essential AAs undergoing ROS-driven oxidation include Leu (to 5-hydroxyleucine), Val (3-hydroxyvaline) and Ile (several products).
Other types of oxidative damages affecting essential AAs involve reactive nitrogen species (RNS). Examples are shown in FIG. 2.
Yet another process detrimental to proteins is a ROS-driven peptide bond cleavage, which is preceded by oxygen free radical-mediated protein oxidation. A hydrogen atom is abstracted from a Cα atom of the polypeptide chain, which then leads to formation of an alkoxyl radical. This can lead either to hydroxyl protein derivative, or to peptide bond cleavage by (1) diamide or (2) α-amidation pathway. This is illustrated in FIG. 3.
Nucleic acids are not normally considered as essential components of the diet, but are also damaged by ROS. An example particularly important for the mitochondrial functioning is the formation of 8-oxy-G, as illustrated in FIG. 4. This leads to mutations in the mitochondrial genome, which is not maintained and repaired as efficiently as the nuclear genome, with detrimental consequences to the efficiency of respiratory processes in the cell. Another cause of degradation is radiation.
The kinetic isotope effect is widely used when elucidating mechanisms and rate-determining stages of chemical and biochemical reactions. The rate of reaction involving C—1H bond cleavage is typically 5 to 10 times faster than the corresponding C—2H(2H=D=deuterium) bond cleavage, due to the two-fold difference in the masses of H and D isotopes. The difference in reaction rates is even higher for tritium (3H or T) as it is 3 times heavier than hydrogen, but that isotope is unstable. The second component of the C—H bond, the carbon atom, can also be substituted for a heavier 13C isotope, but the bond cleavage rate decrease will be much smaller, since 13C is only a fraction heavier than 12C. See Park et al, JACS (2006) 128: 1868-72.
Oxidation reactions are a good example of the isotope effect, as the hydrogen subtraction by an oxidizer is usually a rate-limiting step of the process. Damgaard, Biochemistry (1981) 20: 5662-69, illustrates this: the kinetic isotope effect upon V/K for (1-R)[1-2H2]- and (1-R)[1-3H2]ethanol oxidation by liver alcohol dehydrogenase (ADH) to acetaldehyde, measured at pH 6, was 3 (D(V/K)) and 6.5 (T(V/K)), decreasing to 1.5 and 2.5 respectively at pH 9. Lower than expected rates confirm the discrete role of the non-ADH systems as alternative pathways. In vivo experiments in perfused rat liver, as reported in Lundquist et al, Pharm. & Tox. (1989) 65: 55-62, gave the mean value of D(V/K) of 2.89. Therefore, in all cases the oxidation of deuterated ethanol was substantially slowed down.
Isotopically labelled material has been administered to animals, and also to humans, for diagnostic purposes. Gregg et al, Life Sciences (1973) 13: 755-82, discloses the administration to weanling mice of a diet in which the digestible carbon fraction contained 80 atom % 13C. The additive was 13C-labelled acetic acid. Tissue examination revealed no abnormalities clearly attributable to the high isotopic enrichment.